Episode 3: Life in the Solar System

We met Dr. Oleg Gusev in September 2012 at the Institute of Space and Astronautical Science in Japan. During his presentation, he took a test tube containing something that looked like little red crystals – unmistakably dead and dried out, closely resembling colorful sand grains – and added a little water. By the end of the talk, the grains had transformed into larvae, happily swimming around.

The remarkable species he is studying, officially called Polypedilum vanderplanki, is somewhat similar to mosquitoes and flies, and lives in semiarid central Africa. In order to survive the dry season, which can last up to 8 months without a single drop of rain, the larvae of these special flies have developed the ability to “hibernate” in a completely dried out state. They essentially lose all the water in their body, and just wait around “sleeping” until water returns. In this dried out state, they are basically invincible and immortal. They can stay that way for tens of years with absolutely no water, and they survive temperatures almost down to absolute zero, as well as the heat of a baking oven. Oleg placed them near the failed reactors at Fukushima, and on the outer walls of the International Space Station, and they still came back to life after being returned to a pleasant little pool of water. The only thing they don’t survive, apparently, is being put into alcohol, which fools them into thinking that the water has returned, so they let their guard down too soon.

The Polypedilum vanderplanki is one of the most complex species that can survive under extreme conditions – but by far not the only one. Bacteria (generically known as extremophiles) have been found to thrive in the most adverse places on Earth: around nuclear reactors, in the Mariana Trench, or deep below the Antarctic ice. The common bacterium Streptococcus mitis famously survived a return-trip to the Moon, piggy-backing on the camera of the Surveyor 3 probe, which spent more than two years on the lunar surface before being brought back to Earth by the Apollo 12 mission.

There is in fact a theory called panspermia which says that extremophiles might survive space travel on asteroids and comets, and populate new planets if they happen upon a place with favorable conditions. In 1996, meteorite ALH 84001, which was thought to originate from Mars and fell in Antarctica, made headline news in this context when a magnified image of it revealed the presence of something that looked like fossil martian bacteria. The shape by itself is not enough proof that the structures seen in this meteorite were in fact bacteria and not just something that looked like bacteria but didn’t actually use to be alive – however, thinking that life used to exist on Mars in the past isn’t a crazy thought at all.

Today, the surface of Mars is a cold, frozen desert, with no air to breathe. Its atmosphere is barely one percent of that on Earth. But the pictures sent back by spacecraft orbiting Mars show many dry riverbeds, dry lakes, and a large basin covering most of the northern hemisphere of the planet, surrounded by what looks like shorelines. The pictures show stunning huge canyons, the largest of which, called Valles Marineris, is almost as long as the distance between the East and the West Coast of the United States, and is up to 7 km deep. Mars is also home to the largest volcano in the Solar System, called Olympus Mons, which stands almost three times taller than Mount Everest. The snow-covered peak of this volcano was seen in telescopes long before Mars was visited by spacecraft.

It seems that, billions of years ago, Mars was a different world – more temperate and possibly more pleasant, with an ocean, lakes, and countless rivers. It was an active, dynamic world, with huge volcanoes spewing lava and gasses that kept the atmosphere thick. These gasses were able to maintain a greenhouse effect that kept the planet as warm as Earth, even though it is further from the Sun. Robots that landed on Mars sent us pictures of layered rocks, unmistakeable signs of erosion from flowing rivers in the past. The Mars rovers drilled into these rocks, analyzed their composition, and found that they formed in the presence of flowing water, that might have existed on the surface long enough for simple, single-cell life forms to originate. Two of these robots, Opportunity which landed in 2004 and Curiosity which landed in 2012, are still traveling around the martian surface.

Photo from NASA’s Mars rover Curiosity, taken in August 2012, showing the layered geologic history of the base of Mount Sharp. Credit: NASA/JPL-Caltech/MSSS

On Earth, it took almost 3 billion years for life to evolve into multi-cell organisms, so the conditions on Mars were probably not favorable long enough for complex life forms to develop. But, if extremophiles on Earth are any indication, single cell life forms can be pretty tough. A large amount of bacteria on Earth live deep underground, so if life on Mars ever spread under the surface, it could have survived as the conditions changed and the planet became a frozen desert. Life on Mars might still be lurking under the surface, waiting to be discovered – perhaps near pockets of hot magma in the neighborhood of its giant sleeping or extinct volcanoes.

Like Mars, our other nearest planet, Venus, was off to a great start. It is speculated that the conditions on Venus were similar to Earth around 4 billion years ago, with liquid water on the surface. But then, as the Sun was becoming a grown-up star and the amount of light and heat that it was giving out increased, Venus got a treatment that is worse than the worst nightmare scenarios of global warming. The oceans started to evaporate. Water vapor itself is a greenhouse gas, so this made the temperatures go even higher – so high that the rocks on Venus started to break up and release carbon dioxide into the atmosphere, turning the planet into a veritable oven, with no place to hide and temperatures of almost 500 C. This is already bad enough – but some bacteria might still survive underground, so there was still a glimmer of hope, until it was discovered that Venus doesn’t have tectonic plates. This does not seem concerning at first – but it means all the kind of energy that gets released on Earth through earthquakes is trapped on Venus. This trapped energy accumulates and, every few hundred million years, according to geologists, it can grow so large as to trigger a truly catastrophic event where all the crust is completely destroyed. The crust grows back after these planet-wide eruptions but, alas, any potential life forms that might have survived otherwise pretty harsh conditions will have been entirely wiped out.

So, there are many many factors which determine whether or not a planet is suitable to host life. This unfortunately might make some planets, like Venus, which are reasonably close to the habitable zone of their host star, completely uninhabitable. But there are also factors which make other worlds far out to the edge of the Solar system much more hospitable than their distance from the Sun would suggest.

Worlds as far out as the orbits of Jupiter, Saturn, or Neptune could still harbor life. The moons of these giant gas planets contain lots of water ice, and as these moons are being pulled and squeezed by the tremendous force of gravity of their host planets, their interiors heat up and some of the ice may melt, creating oceans under the surface. There is good evidence for such an ocean under the icy surface of Jupiter’s moon Europa. The Galileo space probe sent us images of Europa showing, in some areas, a smooth icy surface crisscrossed by dark streaks or cracks and, in other areas, a chaotic jumbled terrain. All this suggests geologic activity underneath the crust, probably caused by currents of water slowly shifting the ice above it, much like Earth’s magma is shifting the tectonic plates creating faults and mountains. Under a 10-30 km deep crust of ice on Europa lies an ocean with perhaps twice as much water as all the oceans on Earth. The dark material surrounding the cracks contains organic matter, which means that Europa may have all the necessary ingredients for life as we know it: organic molecules, liquid water, and a source of energy (where the warmth of the Sun, which is hard to come by at that distance, is replaced by heating due to the gravitational push and pull of Jupiter). It may be an exciting world, inhabited by exotic life forms living in a dark subterranean ocean.

Europa may not be the only icy moon capable of hosting and supporting life. Galileo found indications for a subterranean ocean also on Jupiter’s moon Ganymedes, hidden much deeper than the ocean on Europa. Recently, the Cassini space probe also found ice volcanoes and dark organic material on Saturn’s moon, Enceladus. These volcanoes are spewing water with such force that some of it leaves the moon and forms a fresh new shiny ice ring around Saturn. And it appears that the gravitational pulling and squeezing of the icy moons as they move around their orbits can generate enough heat to melt water even at the distance of Neptune. Back in 1989, the Voyager 2 spacecraft flew by Neptune and took images of its moon Triton, finding active volcanoes spewing lava made of water mixed with ammonia.

Several moons of the gas giants in the outer Solar system therefore have what it takes to harbor life, according to our basic understanding of how life originated and what its basic building blocks are. And then there is the curious case of Saturn’s moon, Titan. Titan is the only moon in the Solar system that has a dense atmosphere, and has weather with actual seasons. The Cassini mission revealed a familiar landscape with many surface features similar to those on Earth, such as dunes, rivers, lakes, seas, and deltas. The only exception is that Titan’s rivers and seas and rain and clouds are not made of water, but rather of hydrocarbons, like methane, which are gases on Earth but become liquid at the very low temperatures on the surface of this moon. So, does life need water, or is another liquid good enough? Could we have creatures on Titan with some strange kind of methane-based blood running through their veins, and silicon, which is in the same column in the periodic table, replacing carbon in their DNA? Our current understanding of biology and chemistry says that’s very unlikely. Silicon atoms don’t like each other enough to create huge complex molecules. But perhaps in this case our imagination is really the biggest limit.

Image from NASA’s Cassini spacecraft showing a vast river system on Saturn’s moon Titan. It is the first time images from space have revealed a river system so vast and in such high resolution anywhere other than Earth. The river valley stretches more than 400 kilometers. Scientists deduce that the river is filled with liquid because it appears dark along its entire extent in the high-resolution radar image, indicating a smooth surface. Image credit: NASA/JPL-Caltech/ASI